Blood pressure is one of the most important vital signs used in the assessment of a patient's cardiovascular health. In critical care, it is usually monitored continuously using an invasive fluid-filled monitoring line, also called an arterial line, in which a catheter is inserted into an artery and blood pressure from the artery is transmitted to a blood pressure transducer through fluid-filled tubing 12 to 84 inches long. The arterial pressure as measured by the transducer is displayed on an invasive blood pressure (IBP) monitor. A schematic diagram of such a system is depicted in FIG. 1.
This intra-arterial method not only allows arterial pressure to be monitored continuously on a beat-to-beat basis, but also allows arterial blood to be sampled through the fluid-filled system without the need to cannulate another arterial site. In many hospitals, IBP monitors form part of a central monitoring system in which arterial pressure measurements from patients at various locations in the hospital can be monitored from a central location or from other locations in the hospital. However, this invasive method of monitoring blood pressure is associated with risks of complications such as infection, thrombosis and air embolism.
Noninvasive measurement methods that provide continuous beat-to-beat blood pressure offers an alternative to invasive blood pressure monitoring because they do not carry with them the risk of complications associated with invasive monitoring. The arterial tonometry and vascular unloading methods are two such methods. These methods can be used to measure blood pressure in situations that do not justify the use of invasive means, especially for patients who do not already have an arterial line in place and who also do not require arterial blood sampling.
Commercial noninvasive blood pressure (NIBP) monitors that provide continuous beat-to-beat measurement are mostly standalone monitors that not only cannot or cannot be easily connected to a central monitoring system, but also require a separate monitor to display their waveforms. For example, the Model 7000/CBM-7000 NIBP monitor and the Pilot/BP-508 multiparameter monitor by Colin (Komaki, Japan), both of which provide tonometric blood pressure measurement at the radial artery, are standalone monitors with their own displays and cannot be easily connected to a central monitoring system. The same applies to the Finapres® 2300 NIBP monitor and 2350 NIBP/SpO2 monitors by Ohmeda (now Datex-Ohmeda, Madison, Wis., U.S.A.) and the USM-803 NIBP monitor by UEDA Electronic Works (Tokyo, Japan), all of which measure continuous beat-to-beat blood pressure at a finger using the vascular unloading method. A block diagram showing the main elements of such monitors is presented in FIG. 2.
Another commercial NIBP monitor, the Vasotrac® APM 205A by Medwave (Minneapolis, Minn., U.S.A.), measures blood pressure continually by providing one beat of the pressure waveform for approximately every 15 heartbeats, along with the corresponding systolic, diastolic and mean arterial pressure readings. It uses a modified oscillometric method in which various cycles of increasing and decreasing pressure are applied to the radial artery over a period of 15 heartbeats, and blood pressure is derived from the characteristics of the pressure signal detected by the sensor over this period of time. The main elements of this monitor are the same as those in FIG. 2.
The Vasotrac APM 205A is a standalone monitor. However, the company markets an optional interface, called the NIA V-Line, which connects the Vasotrac APM 205A to an existing IBP monitor to enable the pressure waveform to be displayed on the IBP monitor. A block diagram of an application of this interface is presented in FIG. 3. This interface is associated with U.S. Pat. No. 6,471,646 entitled ARTERIAL LINE EMULATOR. One drawback of this interface is that it requires the use of the Vasotrac itself in order for it to work, so a hospital that only wishes to display the NIBP waveform from the Vasotrac on its existing IBP monitors must purchase the Vasotrac in addition to the interface. This situation adds to the procurement costs for the hospital.